Heat-treating food products for increased shelf life is a well-known and often-employed method. For example, the food product may be various dairy produce such as milk, cream or yoghurt. The heat treatment may take place in a plurality of ways, either directly or indirectly. Indirect methods are, for instance, heating by means of different types of heat exchangers. Of the direct methods, there are two major groups, injection or infusion with steam. By employing a direct method, an extremely rapid heating will be obtained which today is to be sought after since, in order to improve the flavour properties of, for example, milk, it is often the intention to heat to elevated temperatures for brief periods of time.
The present invention relates to an apparatus which employs infusion heating. Infusion entails that a finely-divided liquid is heated in a steam chamber. The principle of heating a liquid, for example a liquid food, by injecting the food into a chamber filled with steam has been known since the early part of the nineteenth century.
The first infusion heating plants were based on the same principle as condensers with which surplus steam from various chemical processes was condensed. The earlier plants thus displayed an infusor which consisted of an autoclave with an inlet for product in the upper region of the autoclave. The inlet discharged in a distributor chamber which divided the incoming product into small, fine liquid droplets. In the bottom of the autoclave there is an outlet for the heated product. The autoclave was further provided with an inlet for steam which, in these early infusors, was located in the lower region of the autoclave.
The first infusors with their steam inlet placed in the lower region of the autoclave thereby automatically obtained a "colder" area in the upper regions of the autoclave. In this "colder" area of the infusor, the uncondensable gases which the product always contains gathers naturally. The uncondensable gases may, for example, be oxygen, nitrogen and carbon dioxide, and these are emitted in connection with the product being heated. A minor quantity of the gases may also come from the steam employed in the process. By obtaining a natural gathering of the gases with a very slight admixture of steam, it is thus easy to lead them off by means of an outlet placed in the upper region of the autoclave.
But since these early infusors suffered from another drawback, this concept was abandoned. The drawback was that when such infusors were used, for example for milk, the steam which, at a low level in the autoclave, is injected in towards the finely-divided liquid droplets caused the droplets to change direction and many of the droplets became stuck to the hot inner walls in the infusor, where burning of the product to such walls was the immediate outcome. The burning of product onto the walls entails major hygienic problems and such burnt product is extremely difficult to wash off.
In order to obviate the problem of burning of the product on the inner walls of the infusor, the steam inlet was placed in the upper region of the infusor. The steam was supplied from above, over the distributor chamber of the product, so that the droplets are not disrupted in the fall down through the steam chamber. But the admission of steam concurrently with the admission of product also suffers from its drawbacks. This type of steam admission may give rise to burning of the product around or in the distributor apertures or distributor gaps displayed by the distributor chamber. This disrupts the heating of the product, the temperature of the product falls, whereupon the steam pressure must be increased and higher temperature differences will occur between product and steam. By designing the distributor apertures such that they have a very thin edge out towards the steam chamber, it is possible to solve the problem in that there will be very thin burnt product layers which are easily broken off before they become disruptive.
But the concurrent admission of steam also resulted in it not being possible to create the "cold" area in the upper region of the autoclave. The uncondensable gases have no natural point of collection. The uncondensable gases may also readily remain trapped in pockets in the downwardly flowing steam and there very readily occurs admixture of steam into the uncondensable gases. This gives a considerably poorer degree of heating of the product, which must be compensated for by an unnecessarily high steam pressure and greater temperature difference between steam and product. The consumption of steam in the process will also be considerably higher, since it is difficult to separate the steam and the uncondensable gases on discharge of the gases.